The present invention relates to synthetic compounds that are active on plants, especially as legume nodulation factors, and also as plant growth stimulators, and to methods for preparing such compounds.
It is known that the process of nitrogen fixing by legumes is based on symbiosis between these plants and soil bacteria, the Rhizobia. The Rhizobium-legume symbiosis produces each year, by means of the root nodules, more ammonium than all of the nitrogen fertilizer industry. This symbiosis thus plays a considerable agronomic role. Legumes are very rich in proteins and produce about one third of the plant proteins consumed worldwide, by virtue of pulses such as soybean, pea, horse bean, groundnut, bean and lupin, and forage plants such as alfalfa and clover.
The formation of nitrogen-fixing nodules starts with an exchange of molecular signals, flavonoids secreted by the plant and nodulation factors (Nod factors) synthesized by the bacterium. These factors consist of an oligosaccharide fragment and a lipid chain attached to this skeleton on the non-reducing end. They have structural attributes (substitutions on the sugars at the two ends and variability of the chain) that make them specific to the legume-bacterium couple.

These lipochito-oligosaccharides (LCO) may be either isolated directly from a particular culture of rhizobia, synthesized chemically, or obtained chemo-enzymatically. Via the latter method, the oligosaccharide skeleton may be formed by culturing of recombinant Escherichia coli bacterial strains in a fermenter, and the lipid chain may then be attached chemically.
Treatment of the seeds of legumes, for instance soybean, with Nod factors at very low concentrations, may result in a large increase in the number of nitrogen-fixing root nodules and a significant increase in the yields, under agronomic conditions. It is thus clear that, in the future, compounds of Nod factor type will be produced industrially for large-scale agronomic use. However, the industrial preparation and conditioning of natural Nod factors presents two types of drawback: (1) the natural Nod factors are difficult to assay via simple methods such as spectrometric methods; (2) they are unstable in the presence of plants or in soils, in particular because they have a —CO—NH— bond that may be broken by plant or microbial enzymes present in the rhizosphere.